Axial Muscles: How They Support the Spine and Breathing

Axial muscles are the muscles that attach to and act on the body’s central axis: the skull, vertebral column, and rib cage. They include the deep back extensors that keep you upright, the intercostals that expand and compress your rib cage during breathing, the abdominal wall muscles, and the small muscles running between your vertebrae. While most fitness culture focuses on the limb muscles that move arms and legs, the axial group quietly handles some of the body’s most essential jobs, from stabilizing the spine under load to powering every breath you take. Their story turns out to be surprisingly rich, stretching from some of the oldest motor circuits in vertebrate evolution to one of modern medicine’s most stubborn problems: chronic low back pain.

What Counts as an Axial Muscle

The simplest way to think about it: axial muscles live along the trunk and neck, while appendicular muscles operate the limbs and the shoulder and hip girdles that connect them to the trunk. The axial group divides broadly into muscles that sit on the back side of the spine and muscles on the front and sides. On the back, the deep intrinsic muscles (the erector spinae group, the multifidus, the semispinalis, and a handful of short rotator and interspinous muscles) run in layers from the base of the skull to the sacrum. On the front and sides, the abdominal muscles (rectus abdominis, the obliques, and the transversus abdominis) form a muscular wall, while the intercostal muscles fill the spaces between the ribs.

A traditional anatomical convention splits the trunk muscles into an epaxial group (dorsal to the spine, innervated by the dorsal branches of the spinal nerves) and a hypaxial group (ventral, innervated by ventral branches). That boundary is not as clean as textbooks sometimes suggest. Research on nerve supply in humans has identified a transitional zone of “midaxial muscles” that receive nerve input from both dorsal and ventral branches, blurring the neat two-part division.¹ The thoracolumbar fascia, a dense sheet of connective tissue that wraps the lower back, develops from a septum that separates epaxial and hypaxial muscle territories during the fifth and sixth weeks of gestation, and it persists as a critical structural interface between them.²

How Axial Muscles Form During Development

Every axial muscle in your trunk traces back to the somites, blocks of tissue that form in pairs along the embryonic spinal cord. The somites give rise to the skeleton, connective tissue, and skeletal muscle of the trunk. Muscle specifically comes from a region of the somite called the dermomyotome, which elongates dorsally and ventrally to populate the back and body wall.³ Molecular markers sort the dermomyotome into its epaxial and hypaxial halves. In bird embryos, the transcription factor En1 labels the epaxial dermomyotome while Sim1 labels the hypaxial portion, and that molecular boundary carries through into the mature muscle and skin.⁴

The switch that turns generic somite cells into committed muscle precursors involves signals from the tissues surrounding the somite. The neural tube, the notochord beneath it, and the overlying skin ectoderm all send molecular cues, including Wnt proteins and Sonic hedgehog, that activate muscle-specifying genes.⁵ A key upstream regulator is Pax-3, a transcription factor that can, on its own, trigger the muscle-determination genes MyoD and Myf-5 in tissue that would not normally become muscle.⁶ Another signaling molecule, beta-catenin, is required within the somite for the dermomyotome and myotome to form properly and for limb muscle precursors to leave the somite and migrate outward.⁷ In other words, the axial muscles do not just happen to sit on the trunk; they are the original muscle lineage, and limb muscles are emigrants that split off later.

Stabilizing the Spine

If you have ever tried balancing a broomstick vertically on your palm, you have a rough sense of the engineering problem the axial muscles solve every waking second. The spinal column is a stack of small bones linked by discs and ligaments. Without active muscular control, it buckles under surprisingly little load. The deepest back muscles, especially the multifidus, are the primary answer to this problem.

The multifidus spans just two to four vertebral segments at a time, giving it fine-grained control over individual motion segments. Its internal architecture, with a high cross-sectional area relative to its fiber length, is built for generating large stabilizing forces rather than large movements.⁸ Musculoskeletal modeling shows that removing the multifidus from a simulated spine dramatically increases compressive loads on the lumbar discs, and the longer erector spinae muscles cannot fully compensate. At 30 degrees of forward bending, for example, removing the multifidus from the model nearly doubled the estimated compressive force at the L4/L5 level.⁹

The back extensors do not work alone. The abdominal muscles and the thoracolumbar fascia form a collaborative system. When the deep abdominals (transversus abdominis and internal oblique) contract, they raise pressure inside the abdomen and simultaneously tension the posterior layer of the thoracolumbar fascia, essentially girdling the paraspinal muscles from behind.¹⁰ That rising intra-abdominal pressure itself offloads the spine. Biomechanical analysis predicts that moderate pressurization reduces spinal compressive force by roughly a fifth during extension effort and up to about 30 percent during lateral bending and axial rotation.¹¹ The practical consequence is that spinal stability depends on the coordinated action of both the back and abdominal axial muscles. Weakness in either group shifts load to the other, or worse, to the passive structures of the spine.

Intra-abdominal pressure also matters dynamically, during real-time lifting. Modeling of transient spine loading during lifts finds that adequate pressure at 75 mmHg increases lumbar stiffness by about a third at higher lift velocities, providing a displacement-limiting safety margin that grows more important as loads and speeds rise.¹² Poor abdominal muscle timing or insufficient activation during a lift can leave the lumbar spine under-stiffened at exactly the wrong moment.

The Breathing Muscles

The intercostal muscles are axial muscles that most people never think about as part of the trunk musculature, but they are indispensable for ventilation. Three layers of intercostals fill each space between adjacent ribs: external, internal, and innermost. Their function depends heavily on where they sit along the rib cage. The external intercostals in the upper, dorsal rib spaces have a strong inspiratory action, expanding the chest during inhalation, but that advantage reverses as you move toward the lower, ventral rib spaces. The internal intercostals show the opposite gradient, with the lower segments powering forced exhalation while the parasternal portions of the upper internal intercostals actually assist with inspiration.¹³

Rib movement itself is not uniform. Ribs two through six swing upward and forward when you inhale, increasing the front-to-back diameter of the chest (often called the pump-handle motion), while ribs seven through ten flare outward to widen the chest side to side (the bucket-handle motion).¹⁴ The abdominal wall muscles, though usually discussed in the context of trunk stability, also participate directly: they relax during inspiration to let the diaphragm descend fully, and they contract during forced expiration to push the diaphragm back up. A tight corset or rigid abdominal brace can actually inhibit full inspiration by preventing that abdominal relaxation.

Nerve Supply and Motor Control

The way axial muscles are wired reflects their developmental origins. A spinal nerve exits the vertebral column and immediately splits into a dorsal ramus and a ventral ramus. The dorsal ramus supplies the epaxial back muscles; the ventral ramus supplies the hypaxial muscles of the body wall, and also sends branches out to the limbs via the nerve plexuses. A three-component model of spinal nerve branching describes this arrangement: segmental branches for the primaxial (trunk-proper) muscles, intramural plexus branches for the body wall, and extramural branches for the girdle and limb muscles.¹⁵ This segmental innervation pattern is ancient and highly conserved. Developmental studies in mice show that the primary branches of the spinal nerve establish themselves early, with the dorsal ramus initially supplying cutaneous branches to the skin of the back before innervating the deep muscles.¹⁶

Axial muscle coordination for locomotion is largely governed by central pattern generators in the spinal cord: networks of neurons that can produce rhythmic motor output even without input from the brain. In salamanders, electrophysiological recordings from isolated spinal cords show that the mid-trunk spinal networks can generate coordinated axial motor patterns on their own, and even a half-cord is sufficient.¹⁷ This explains why axial locomotion patterns are so robust and stereotyped across vertebrates: the circuitry does not require much top-down supervision.

Fiber Type and Sensory Richness

Because the paraspinal muscles must sustain contraction for hours at a stretch to keep you upright, they are loaded with slow-twitch (Type I) fibers, the fatigue-resistant fibers suited for endurance rather than explosive power.¹⁸ This high slow-twitch proportion distinguishes them from most limb muscles, which tend toward a more mixed or fast-twitch-biased profile depending on function.

The axial muscles of the neck are also remarkably rich in muscle spindles, the sensory organs that detect stretch and relay position information to the nervous system. The deep neck muscles have an extremely high muscle spindle density, far exceeding what you find in limb muscles of comparable size.¹⁹ This makes the deep neck muscles function almost as much as sensory organs as motor ones: they feed continuous, high-resolution proprioceptive data about head position to the brain. Disruption of this sensory input, from whiplash injury, for example, helps explain why neck trauma can produce dizziness, impaired balance, and disorientation out of proportion to visible structural damage.

Axial Muscles and Low Back Pain

Chronic low back pain is one of the most common reasons people visit a doctor, and the axial muscles are deeply involved. Multifidus atrophy, where the muscle shrinks and becomes infiltrated with fat, is consistently associated with non-specific chronic low back pain.²⁰ The pattern of wasting is localized rather than generalized: studies find that the multifidus at the L5 vertebral level is significantly smaller in people with chronic low back pain compared to healthy controls, and those same individuals also show a reduced ability to voluntarily contract the atrophied muscle.²¹ MRI case series have documented progressive fatty replacement of the multifidus over years of chronic symptoms.²²

What makes this worse is that spinal surgery itself can damage the axial muscles. Post-operative atrophy of the paraspinal muscles, including the erector spinae as well as the multifidus, is a recognized consequence of posterior lumbar surgery. Because the erector spinae muscles sit laterally and are not directly cut during the procedure, their volume loss is thought to result from denervation and prolonged immobilization rather than direct surgical trauma.²³ Aging compounds the problem: paraspinal muscle atrophy and fatty infiltration are features of age-related sarcopenia in the trunk, contributing to both functional limitation and chronic back pain in older adults.²⁴

MRI-based measures of fat infiltration in the paraspinal muscles are becoming increasingly important in clinical assessment. Quantitative approaches, such as calculating the ratio of fat area to total muscle cross-section, have been shown to correlate with pain and physical function. Fat infiltration tends to increase from the upper to the lower lumbar spine, and greater infiltration is associated with worse outcomes in conditions like lumbar spinal stenosis.²⁵ A muscle-fat index, calculated from MRI signal intensities, has proven reliable enough to be used across multiple clinical centers.²⁶

When Axial Muscles Fail Dramatically

Beyond garden-variety low back pain, certain neurological and inflammatory conditions can devastate the axial muscles in ways that produce striking postural deformities. Camptocormia, a severe involuntary forward bending of the trunk, and dropped head syndrome, where the neck extensors become too weak to hold the head up, are two of the most recognizable. These conditions can result from Parkinson’s disease, dystonia, motor neuron disease, myositis, and certain muscular dystrophies.²⁷

When axial muscle weakness is the first sign of an inflammatory myopathy (a group of autoimmune muscle diseases), the diagnostic picture can be confusing because clinicians expect limb weakness to appear first. Retrospective analysis suggests that the inflammatory myopathies most likely to present initially with axial involvement are inclusion body myositis, overlap myositis, and dermatomyositis. During follow-up, axial weakness eventually appears in a majority of patients with some of these subtypes, which may indicate that early axial involvement is underdiagnosed.²⁸

Evolutionary Roots of Axial Musculature

Axial muscles are, evolutionarily speaking, the original locomotor system. In fish, the entire body is driven forward by waves of axial muscle contraction passing down the trunk. Limbs came later, and when they did, the role of the axial muscles shifted but never disappeared. A review of axial muscle evolution across vertebrates shows that the ancestral function of mobilization has been retained in every lineage, but land-dwelling animals added a new job: global stabilization of the trunk against gravity and the forces generated by the limbs.²⁹

The transition from water to land is a particularly interesting chapter. Lungfish, which are among the closest living relatives of the first land vertebrates, use a standing-wave pattern of axial muscle activation when they move on land that closely resembles what salamanders use during terrestrial locomotion.³⁰ This is a remarkable finding because it suggests that some of the neural wiring for land locomotion was already in place before limbs evolved into weight-bearing structures. The axial muscles were ready for land before the legs were.

Axial muscle architecture continues to be shaped by locomotor ecology in living animals. In snakes, which abandoned limbs entirely and returned to axial-only locomotion, the architecture of trunk muscles varies with habitat. Aquatic snakes develop heavier semispinalis-spinalis and longissimus dorsi muscles with greater cross-sectional area, reflecting the demands of pushing through a dense, viscous medium. Arboreal snakes, by contrast, have lighter trunk muscles, which may reduce the energetic cost of climbing.³¹

Training the Axial Muscles

One underappreciated fact about the axial muscles is that they do not hypertrophy easily from general exercise. The postural muscles of the neck illustrate this well. A study comparing resistance-training protocols found that general resistance exercise (without specific neck work) produced no measurable increase in neck muscle cross-sectional area, while a program that included targeted neck extension exercises increased the cross-sectional area of the deep neck extensors (splenius capitis, semispinalis capitis, and semispinalis cervicis) by roughly a quarter each.³² The researchers noted that the everyday postural load on these muscles in upright humans is modest enough that it does not provide a meaningful training stimulus on its own.

The same principle applies, broadly, to the lumbar multifidus and the deep paraspinal group. Compound lifts like squats and deadlifts load the trunk and require axial stability, but whether they provide sufficient targeted stimulus to reverse established multifidus atrophy is less clear. Rehabilitation programs for chronic low back pain frequently include exercises specifically designed to activate the multifidus and transversus abdominis in isolation before progressing to integrated movement. The rationale traces directly back to the evidence that multifidus atrophy is localized and involves a reduced capacity for voluntary contraction: you cannot strengthen a muscle you cannot recruit, so relearning activation often has to precede loading.

The thoracolumbar fascia adds a layer of complexity to trunk training. Because it mechanically couples the deep abdominals to the paraspinal muscles, exercises that combine abdominal bracing with back extensor contraction tension the fascia in a way that girdles and supports the lumbar spine.³³ The fascia also transfers force between major muscle groups: contraction of the latissimus dorsi, gluteus maximus, erector spinae, and even the biceps femoris all displace and tension the posterior layer of the thoracolumbar fascia.³⁴ This means the axial muscles participate in force transmission chains that run from the shoulders through the trunk to the legs, and training programs that ignore the trunk are leaving a critical link underdeveloped.